The waste disposal problem currently encountered by public, private, and government interests has numerous facets. The variables include waste streams comprised of a broad spectrum of wastes, coupled with varying cost and final effluent requirements. For example, one waste stream may include sludge from a municipal sewage treatment facility with its high bacterial counts, while another may contain a mixture of hydrocarbons including various halogens, sulfur and light metal elements such as sodium. Furthermore, many waste streams may also include hazardous heavy metal ions such as lead or chromium. A typical waste stream may well include any one or more of the aforementioned wastes as well as various particulates all in differing levels of concentrations. It quickly becomes apparent that an efficient, broad spectrum waste treatment process would be of great utility in addressing this problem. The present disclosure sets forth both a method and apparatus that can handle the wide variety of wastes described above in a cost and energy efficient manner.
In the past, a number of systems have been proposed for handling wastes of this nature. One group of such systems uses the unique properties of water when it is in its supercritical state (above 374.degree. C. and 3,206 psi). In this high-energy, dense-vapor form, water dissolves normally insoluble organics and permits the separation and disposal of inorganics, for example, metals. The end products are carbon dioxide, salt, water, and heat. This technology is covered in U.S. Pat. Nos. 4,113,446, 4,338,199, and 4,543,190. In the process described therein, the waste solution is pressurized and fed to the reactor along with compressed oxygen. Alkaline material may be injected into the feed stream in order to neutralize any acids. The combined streams are raised to at least the critical point, where the rapid oxidation begins. Part of the effluent may be then recycled to or heat exchanged with the raw stream. The reaction products of salt, superheated water, and carbon dioxide are cooled and discharged at atmospheric conditions. The heat released by cooling the effluent can be used for feed preheating, steam generation, power generation, or for lower-level heating requirements. Salts in the incoming feed as well as those generated in the process are removed as a brine. The salts have very low solubility in the supercritical water but are typically highly soluble in cool water. The process first separates the salts and then redissolves them in a cool brine as a means of transporting them from the reactor. In the process described above, the feed material is oxidized in one vessel while the solids are separated out in a subsequent vessel. There is a substantial risk of solids deposition and system plugging in passing to this second vessel.
Another process for conducting chemical reactions involving organic and inorganic waste streams at supercritical conditions is described in U.S. Pat. No. 4,594,164. In that process, continuously flowing water contaminated with organic and inorganic materials is fed to the top of a downdraft column of a hydraulic column reactor, and conducted to the bottom thereof to a reaction chamber. Supercritical water conditions are created in the reaction chamber in order to oxidize the waste elements in the water. The reacted fluid is conducted back to the surface over a spiral baffle or rib and through a series of annuli. Spinning of the rising fluid caused by the spiral baffle induces centrifugal separation of the fluid into various strata of differently weighted components which travel up separate annuli. The resultant materials are removed for further solids separation, treatment and disposal. A similar system is described in U.S. Pat. No. 4,564,458. In that system, a deep well is utilized to form a reaction chamber for combustible waste in water. A stream of water borne combustible waste is delivered into the deep well, one sufficiently deep to obtain a pressure and temperature in a bottom located reaction chamber at which the water becomes supercritical. A pipe is used to deliver oxygen under pressure to the reaction chamber for combusting oxygen dissolved in the supercritical water with the waste materials. The resulting effluent is conducted upwards through a separate updraft column back to the surface for further treatment and disposal.
Problems likely to be encountered in such systems would include corrosion, and possible system leakage into the surrounding area. While corrosion in all supercritical water reaction systems is a problem, the inaccessibility of the underground components greatly increases the difficulty of monitoring and correcting any corrosion problems. Moreover, any significant corrosion could lead to leaks into the surrounding areas. Another problem with these systems is their handling of solids. Inorganic salts would cause scaling on the walls of the system pipes, leading to reduced heat transfer and possible plugging. Frequent shutdowns would be required for system cleaning. The entraining of any solids into the flow of the effluent stream may be another potential problem for the deep well system. Deep well systems must maintain low velocities to minimize friction losses. The design velocities of deep well systems are in the 1-20 feet/sec range, which is unlikely to be high enough to entrain all solids to the top of the effluent stream. Solids would build up near the bottom of the effluent pipe and if these are insoluble in water and acid, they may be very difficult to remove.